1. Field of the Invention
The present invention relates to an optical pick-up and an optical recording system for performing a process such as a recording process or a reproducing process to an optical recording medium such as an optical disk.
2. Description of the Related Art
As an optical disk which is recently available, a compact disk (CD) based on the compact disk standards (CD standards) is known. The substrate of the compact disk has a thickness of 1.2 mm, and a track pitch formed on the surface of the substrate to record information is 1.6 mm. In order to reproduce the CD, an optical pick-up comprising a laser diode for emitting a laser beam having a wavelength of 0.78 mm and an objective lens having a numerical aperture of 0.45 is used. In recent years, various developments were made to increase the recording density of the optical disk, and it is examined in the developments to increase the numerical aperture of the objective lens. When the numerical aperture of the objective lens is increased, the optical resolution is improved. For this reason, the linear recording density on the optical disk can be increased, and the pitch of recording tracks can be decreased. Therefore, the recording density can be considerably increased.
However, when the numerical aperture of the objective lens is increased, a phenomenon that the aberration of a light spot focused by the objective lens occurs. More specifically, due to axial runout of the optical disk and axial runout of a turntable which has the optical disk attached thereto and is rotated, inclination (so-called tilt) of the optical disk and the objective lens occurs, and a coma aberration occurs in this light spot due to this tilt. For this reason, when only the numerical aperture is increased, focusing performance is not improved, and an optical disk having a high recording density cannot be realized.
For this reason, it is considered that, by decreasing the substrate of the optical disk in thickness, the numerical aperture of the objective lens is increased while preventing degradation of an aberration (coma aberration) caused by tilt. On the bas is of this consideration, standardization of the high-density optical disk advances. One of the standards is the standard of a digital video disk (DVD). In this DVD standard, the thickness of the optical disk is set to 0.6 mm, and the track pitch is set to 0.74 mm. To this DVD, a recording or reproducing operation is performed by using an optical pick-up for irradiating a laser beam having a wavelength of 0.65 mm or 0.635 mm through an objective lens having a numerical aperture of 0.6.
When t he optical disk having a thin substrate based o n this standard is used, a recording density can be increased. However, since the objective lens of the DVD optical pick-up is suitably designed for a thin optical disk, a spherical aberration increases when the objective lens is applied to an optical disk having a conventional thick substrate, thereby degrading imaging performance. For this reason, the DVD optical pick-up cannot perform a recording/reproducing operation to a CD. Further development has been made to prevent the above disadvantages. It is considered that a laser beam emitted from the optical pick-up is split into two focusing points to be converged, thereby coping with optical disks having different thicknesses. For example, Japanese Unexamined Patent Publication No. 7-65407 describes the following technique. That is, a light flux emitted from a laser diode is split by adding a prism or a hologram to an optical system, and the light fluxes are converged by different optical lengths, respectively, thereby forming two beam spots. In Japanese Unexamined Patent Publication No. 7-98431, a composite objective lens is formed by combining a deflective objective lens and a hologram lens to form two beam spots.
However, in the optical pick-up described above, in order to make it possible to process optical disks having different thicknesses, one laser beam is split into two beam spots to be converged. For this reason, the energies of the beams converged to the respective beam spots decrease. Therefore, since the intensity of an reflected beam reflected from the optical disk decreases, the sensitivity of the light detector must be improved, or a laser diode having a large laser output power must be employed. For this reason, the optical pick-up increases in size and power consumption, and the manufacturing cost of the optical pick-up also increases. Although an optical disk having a high recording density requires a focus servo or tracking servo having higher precision, a large optical pick-up requires high power to follow a focusing error or a tracking error, and an actuator for performing these servo operations increases in size. In this manner, the optical pick-up having two focusing points and a large size requires high power consumption. For this reason, an optical recording system employing the optical pick-up is also large and expensive, and the power consumption of the optical recording system more increases.
Since the conventional optical pick-up in which two focusing points are always formed by the objective lens always has two beam spots converged by the objective lens, for example, a focus drawing operation performed to a thick optical disk may be erroneously performed at a beam spot for reproducing a thin optical disk. Therefore, a mechanism or control for preventing a focus drawing operation from being performed at the beam spot for reproducing the thin optical disk must be additionally arranged. Furthermore, when a thick optical disk is to be reproduced, there is a beam spot for reproducing a thin optical disk. This beam spot is diffused because it is not focused on the recording surface of the optical disk. The beam spots may be finally incident on photo detectors used for signal detection as stray rays to make noise. For these reasons, a complicated focus drawing mechanism causes the optical pick-up to increase in size and cost, and measures to prevent erroneous operation or stray rays must be taken. These measures are another factor that increases the optical pick-up's size and cost.
By utilizing conventional techniques, laser beam sources and optical elements which are respectively suitable for an optical disk having a low recording density and an optical disk having a high recording density can be independently arranged. However, such an optical pick-up requires a large number of independent laser beam sources and optical elements which further increases its size and cost.
As an optical pick-up which is reduced in size, an optical pick-up using a surface emitting laser array as light sources is disclosed in Japanese Unexamined Patent Publication No. 7-192291. By using an advantage that a surface emitting laser can be easily formed on a two-dimensional array, light sources in which five surface emitting lasers are formed on a single substrate in the form of an array is used (i.e., a surface emitting laser for detecting an information signal, two surface emitting lasers for detecting a focusing error signal, and two surface emitting lasers for detecting a tracking error signal). However, this optical pick-up is used only to obtain a small size, but is not used to reproduce optical disks having different substrate thickness or different track pitches.
Therefore, it is an object of the present invention to provide a compact optical pick-up which can be easily controlled and in which beam spots are appropriately formed on optical disks having both a high recording density and a low recording density, and being different in thickness, without splitting a laser beam into two focusing points to be converged. It is another object of the present invention to provide an optical pick-up which can stably perform a recording/reproducing process to optical disks having different thicknesses, while having a small size, simple arrangement, low power consumption, and low cost.